Journal
JOURNAL OF FIELD ORNITHOLOGY
Volume 84, Issue 3, Pages 270-286Publisher
WILEY
DOI: 10.1111/jofo.12026
Keywords
BACI analysis; bird surveys; density; detection probability; distance sampling
Categories
Funding
- National Fish and Wildlife Foundation
- National Council for Air and Stream Improvement, Inc.
- U.S. Fish and Wildlife Service
- U.S. Forest Service Monongahela and Daniel Boone National Forests
- MeadWestvaco Corporation
- Kentucky Department of Fish and Wildlife Resources
- Ohio Division of Wildlife through the USFWS State Wildlife Grant Program
- Tennessee Wildlife Resources Agency (TWRA)
- University of Tennessee Department of Forestry
- Wildlife and Fisheries
- Nature Conservancy through a USFWS Habitat Conservation Planning
- TWRA
- West Virginia Division of Natural Resources (WVDNR) Wildlife Diversity Program
- U.S. Geological Survey West Virginia Cooperative Fish and Wildlife Research Unit
- IACUC protocols from Ohio State University [2004A0047, 2007A0015, 2010A0003]
- West Virginia University [04-0302, 07-0303]
- University of Tennessee [561]
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Point counts are commonly used to assess changes in bird abundance, including analytical approaches such as distance sampling that estimate density. Point-count methods have come under increasing scrutiny because effects of detection probability and field error are difficult to quantify. For seven forest songbirds, we compared fixed-radii counts (50 m and 100 m) and density estimates obtained from distance sampling to known numbers of birds determined by territory mapping. We applied point-count analytic approaches to a typical forest management question and compared results to those obtained by territory mapping. We used a before-after control impact (BACI) analysis with a data set collected across seven study areas in the central Appalachians from 2006 to 2010. Using a 50-m fixed radius, variance in error was at least 1.5 times that of the other methods, whereas a 100-m fixed radius underestimated actual density by >3 territories per 10 ha for the most abundant species. Distance sampling improved accuracy and precision compared to fixed-radius counts, although estimates were affected by birds counted outside 10-ha units. In the BACI analysis, territory mapping detected an overall treatment effect for five of the seven species, and effects were generally consistent each year. In contrast, all point-count methods failed to detect two treatment effects due to variance and error in annual estimates. Overall, our results highlight the need for adequate sample sizes to reduce variance, and skilled observers to reduce the level of error in point-count data. Ultimately, the advantages and disadvantages of different survey methods should be considered in the context of overall study design and objectives, allowing for trade-offs among effort, accuracy, and power to detect treatment effects.
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